1. Field of the Invention
This invention generally relates to signal transmission and processing, and specifically to a method and apparatus for providing efficient telephone and data service to Cable TV subscribers through the use of an improved method of Time Division Multiple Access (TDMA) communications.
2. Discussion of the Related Art
Cable Antenna Television (CATV) Networks provide an established network infrastructure for high speed communications. Traditionally, CATV networks operate solely as broadcast networks. Information, in the form of television programs, is transferred one way, from the CATV headend to the CATV subscribers. With the advent of advanced technologies and changes in the regulatory restrictions, CATV network operators have an opportunity to use their network infrastructure to provide additional services to their subscribers (see FIG. 1). These services will include, for example, telephone service, internet access, video on demand, etc. These additional services will require the networks to handle high speed, high bandwidth communications both to and from the subscriber, and, to be successful, will be required to be highly robust and reliable.
CATV networks differ from other high bandwidth networks. High bandwidth cable and fiber networks are employed for a variety of data communication systems, but such systems are typically characterized as being relatively close geographically, e.g. Local Area Networks (LANs). CATV networks often span large geographic regions, and an effective CATV network must accommodate subscriber locations with significantly different transmission paths to the CATV headend.
Consider, for example, attempting to synchronize the transmission of information from a variety of subscriber locations. A common technique for communications from multiple sources is Time Division Multiple Access (TDMA), wherein each transmitter is assigned a time slot relative to a synchronizing timing signal sent from the receiver. It is imperative in TDMA that the communications from these multiple sources do not overlap in time. A sufficient gap between assigned time slots must be allowed to accommodate any variances in the timings of the signals from the various transmitters. This gap between time slots is a significant factor in the determination of the overhead, or inefficiency, of a TDMA network. The inefficiency can be measured as the ratio of the gap time to the slot time. If each transmitter is allocated a relatively long slot time, then long gap times can be tolerated. However, a long slot time per transmitter results in a long interval between slot times for each transmitter. Long periods of time between transmissions increases the cost of memory to buffer the data between transmissions, and, in some application, such as voice, can cause intolerable delays. Thus, short and frequent slots are preferred to long and infrequent slots, albeit at the cost of decreased efficiency caused by gap times.
In most high speed networks, the variance among the transmitters, and hence the gap time, can be controlled to be less than a few microseconds. However, in a typical CATV network, the source transmitters are located at various geographic locations relative to the cable headend location. It would not be unusual, for example, to have one subscriber located within a mile of the headend, and another located twenty or more miles away from the headend. At a typical speed of transmission of 125,000 miles per second over a fiber optic cable, it would take a signal over 160 microseconds to reach a device located twenty miles away, and less than 8 microsecond to reach a device one mile away. Thus, the synchronizing timing signal received by each of these transmitters would have a variance of over 150 microseconds. Additionally, this variance in time would be incurred for signals transmitted back to the headend, such that the variance between signals from devices located twenty miles apart for a round trip transmission would amount to over 300 microseconds. The time slot allocated for each source transmitter in this environment would necessarily be set to be at least 300 microseconds, to allow for these variances (see FIG. 7). Even if a 300 microsecond gap time could be tolerated, however, setting the time slot to this specific gap time will limit the geographic extent of the CATV network to the aforementioned twenty miles, thereby limiting the growth of such a network to this predetermined distance. To avoid such a constraint, the time slot per transmitter would be specified at some maximum value, corresponding to the variance to be expected at the maximum predictable extent of the network. This maximum value would also include an allowance for the actual cable length and routing between locations, rather than the geographic distance. It would not be unreasonable in a typical CATV environment to specify a 500 microsecond or more time slot for each transmitter to accommodate the variance in transmission delay time among transmitters. Such an extensive time slot, relative to a short message length, will result in a very inefficient, or very costly, TDMA network. Shorter time slots, absent this invention, would require additional headends and geographically smaller networks.